1. Technical Field
The present invention relates to an image forming apparatus of the electrophotographic system, and more particularly to an improvement of an image forming apparatus in which a belt-like transferring member is driven by a drum-like image carrier.
2. Related Art
In the related art, an image forming apparatus in which a belt-like member is used as a component is known. Among image forming apparatuses of this kind, for example, there is an apparatus having: a photosensitive drum that is a drum-like image carrier that is rotated; a charger, a writer, a developer, a primary transfer device, and a cleaner that are arranged around the photosensitive drum, and comprising an intermediate transfer belt that is a belt-like member that is opposed to the photosensitive drum while being contacted therewith, and that is rotated at the same peripheral speed as the photosensitive drum. In such an image forming apparatus, the intermediate transfer belt is rotated in a predetermined direction. A position detecting sensor that optically detects a positioning marker is disposed on the intermediate transfer belt, and is formed by a reflective material disposed in an area where the positioning marker passes.
When the image forming apparatus is to form an image, during rotations of the photosensitive drum and the intermediate transfer belt, the image forming operation is started at a constant timing from detection of the positioning marker by the position detecting sensor. When the image forming operation is ended, a driving motor that rotates the intermediate transfer belt is stopped so that the intermediate transfer belt is also stopped. At this time, in order to make constant the so-called first print out time (hereinafter, referred to also as FPOT) from reception of instructions for forming the next image to completion of an output of the image, the driving motor is stopped after an elapse of a preset time after detection of the positioning marker by the position detecting sensor so that the intermediate transfer belt is always stopped at the same position.
However, since the intermediate transfer belt is stretched by plural holding rolls, there arises a possibility that an image failure such as a white band occurs in the following manner. When the intermediate transfer belt is always stopped at the same position as described above, a curl due to the holding rolls occurs over time in portions that are wound around the holding rolls.
In order to meet recent requests for cost reduction and miniaturization, an image forming apparatus of the electrophotographic system is known in that an intermediate transfer belt is driven by rotation of a photosensitive drum. In an image forming apparatus having such a configuration, the intermediate transfer belt is stretched by plural roll portions, and a part of the outer periphery of the belt is in surface contact with the photosensitive drum so as to cover the peripheral face of the photosensitive drum. The rotating force of the photosensitive drum is used as a force of driving the intermediate transfer belt by a contact frictional force of the surface-contact portion. In the image forming apparatus, since the intermediate transfer belt is in surface contact with the drum-like photosensitive member so as to cover a part of the surface of the photosensitive drum, a concave curl is formed in the intermediate transfer belt. When the concave curl portion is circularly moved to again contact with the surface (peripheral face) of the photosensitive drum, the portion acts so as to separate from the surface. In a configuration where only a countermeasure of stopping the intermediate transfer belt at various positions is taken as in the related art, therefore, slippage easily occurs between the intermediate transfer belt and the surface of the photosensitive drum, and hence a notable problem is caused by an image failure such as color misalignment.